"Pea
hybrids form germinal and pollen cells that in their composition
correspond in
equal numbers to all the constant forms resulting from the combination
of traits
united through fertilization."

Gregor Johann Mendel was born
on July 22, 1822 to
peasant parents in a small agrarian town in Czechoslovakia. During his
childhood
he worked as a gardener, and as a young man attended the Olmutz
Philosophical
Institute. In 1843 he entered an Augustinian monastery in Brunn,
Czechoslovakia.
Soon afterward, his natural interest in science and specifically
hereditary
science led him to start experiments with the pea plant.Mendel's attraction for scientific research was based on his
love of
nature in general. He was not only interested in plants, but also in
meteorology
and theories of evolution. However, it is his work with the pea plant
that
changed the world of science forever.

His
beautifully designed experiments with pea plants were the first to
focus on
the numerical relationships among traits appearing in the progeny of
hybrids.His interpretation for this phenomenon was that material and
unchanging
hereditary elements undergo segregation and independent assortment.These elements are then passed on unchanged (except in
arrangement) to
offspring thus yielding a very large, but finite number of possible
variations.

Mendel often wondered how plants obtained atypical characteristics. On
one of
his frequent walks around the monastery, he found an atypical variety
of an
ornamental plant. He took it and planted it next to the typical
variety. He grew
their progeny side by side to see if there would be any approximation
of the
traits passed on to the next generation. This experiment was designed
to
support or to illustrate Lamarck's views concerning the influence of
environment
upon plants.He found that the
plants' respective offspring retained the essential traits of the
parents, and
therefore were not influenced by the environment. This simple test gave
birth to
the idea of heredity.

Overshadowing the creative brilliance of Mendel's work is the fact that
it was
virtually ignored for 34 years. Only after the dramatic rediscovery of
Mendel's work in 1900 (16 years after Mendel's death) was he rightfully
recognized as the founder of genetics.1

Why
Peas?

Pisum
sativum

Mendel was well aware that there were certain preconditions that had to
be
carefully established before commencing investigations into the
inheritance of
characteristics. The parental plants must be known to possess constant
and
differentiating characteristics.To
establish this condition, Mendel took an entire year to test "true
breeding" (non-hybrid) family lines, each having constant characteristics.The experimental plants also needed to produce flowers that
would be easy
to protect against foreign pollen.The
special shape of the flower of the Leguminosae family, with
their
enclosed styles, drew his attention.On
trying several from this family, he finally selected the garden pea
plant (Pisum
sativum) as being most ideal for his needs.Mendel
also picked the common garden pea plant because it can be grown in
large numbers
and its reproduction can be manipulated. As with many other
flowering
plants, pea plants have both male and female reproductive organs.
As a
result, they can either self-pollinate themselves or cross-pollinate
with other
plants. In his experiments, Mendel was able to selectively
cross-pollinate
purebred plants with particular traits and observe the outcome over
many
generations. This was the basis for his conclusions about the
nature of
genetic inheritance.3

Mendel
observed seven pea plant traits that are easily recognized in one of
two forms:

1.Flower
color: purple or white

2.Flower
position: axial or terminal

3.Stem
length: long or short

4.Seed
shape: round or wrinkled

5.Seed
color: yellow or green

6.Pod
shape: inflated or constricted

7.Pod
color: green or yellow

Mendel's
Law of Segregation

Mendel's
hypothesis essentially has four parts. The first part or "law" states
that, "Alternative versionsof
genes account for variations in inherited characters." In a nutshell,
this is
the concept of alleles. Alleles are different versions of genes that
impart the
same characteristic.For example,
each pea plant has two genes that control pea texture.There are also two possible textures (smooth and wrinkled) and
thus two
different genes for texture.
The second law
states that, "For each
character trait (ie: height, color, texture etc.) an organism inherits
two
genes, one from each parent." This
statement alludes to the fact that when somatic cells are produced from
two
gametes, one allele comes from the mother, one from the father. These
alleles
may be the same (true-breeding organisms), or different (hybrids).

The
third law, in relation to the second, declares that, "If the two
alleles
differ, then one, the dominant allele, is fully expressed in the
organism's
appearance; the other, the recessive allele, has no noticeable effect
on the
organism's appearance."

The
fourth law states that, "The two genes for each character segregate
during
gamete production." This is
the last part of Mendel's generalization. This references meiosis when
the
chromosome count is changed from the diploid number to the haploid
number. The
genes are sorted into separate gametes, ensuring variation.This sorting process depends on genetic "recombination."
During this time, genes mix and match in a random and yet very
specific
way.Genes for each trait only
trade with genes of the same trait on the opposing strand of DNA so
that all the
traits are covered in the resulting offspring.For example, color genes do not trade off with genes for texture.Color genes only trade off with color genes from the opposing
allelic
sight as do texture genes and all other genes.The result is that each gamete that is produced by the parent is
uniquely
different as far as the traits that it codes for from every other
gamete that is
produced.For many creatures, this
available statistical variation is so huge that in all probability, no
two
identical offspring will ever be produced even given trillions of years
of time.

So,
since a pea plant carries two genes, it can have both of its genes be
the same.Both genes could be "smooth" genes or they could both be
"wrinkled" genes.If both genes
are the same, the resulting pea will of course be consistent.However, what if the genes are different or "hybrid"?One gene will then have "dominance" over the other "recessive"
gene.The dominant trait will then
be expressed.For example, if the
smooth gene (A) is the dominant gene and the wrinkle gene (a) is the
recessive
gene, a plant with the "Aa" genotype will produce smooth peas.Only an "aa" plant will produce wrinkled peas.For instance, the pea flowers are either purple or white.Intermediate colors do not appear in the offspring of these
cross-pollinated plants.

The
observation that there are inheritable traits that do not show up in
intermediate forms was critically important because the leading theory
in
biology at the time was that inherited traits blend from generation to
generation (Charles Darwin and most other cutting-edge scientists in
the 19th
century accepted this "blending theory.").Of course there are exceptions to this general rule.Some genes are now known to be "incompletely dominant." In
this situation, the "dominant" gene has incomplete expression
in the
resulting phenotype causing a "mixed" phenotype.For example, some plants have "incomplete dominant" color genes
such
as white and red flower genes.A
hybrid of this type of plant will produce pink flowers.Other genes are known to be "co-dominant" were both alleles are
equally expressed in the phenotype.An
example of co-dominant alleles is human blood typing.If a person has both "A" and "B" genes, they will have an
"AB" blood type.Some traits
are inherited through the combination of many genes acting together to
produce a
certain effect.This type of
inheritance is called "polygenetic." Examples
of polygenetic inheritance are human height, skin color, and body form.In all of these cases however, the genes (alleles) themselves
remain
unchanged.They are transmitted
from parent to offspring through a process of random genetic
recombination that
can be calculated statistically.For
example, the odds of a dominant trait being expressed over a recessive
trait in
a two-gene allelic system where both parents are hybrids are 3:1.If only one parent is a hybrid and the other parent has both
dominant
alleles, then 100% of the offspring will express the dominant trait.If one parent has both recessive alleles and the other parent is
a
hybrid, then the offspring will have a phenotypic ratio of 1:1.

Mendel's
Law of Independent Assortment

The
most important principle of Mendel's Law of Independent Assortment is
that the
emergence of one trait will not affect the emergence of another. For
example, a
pea plant's inheritance of the ability to produce purple flowers
instead of
white ones does not make it more likely that it would also inherit the
ability
to produce yellow peas in contrast to green ones. Mendel's
findings
allowed other scientists to simplify the emergence of traits to
mathematical
probability (While mixing one trait always resulted in a 3:1 ratio
between
dominant and recessive phenotypes, his experiments with two traits
showed
9:3:3:1 ratios).

Mendel
was so successful largely thanks to his careful and nonpassionate use
of the
scientific method. Also, his choice of peas as a subject for his
experiments was
quite fortunate.Peas have a
relatively simple genetic structure and Mendel could always be in
control of the
plants' breeding. When Mendel wanted to cross-pollinate a pea plant he
needed
only to remove the immature stamens of the plant. In this way he was
always sure
of each plants' parents. Mendel made certain to start his experiments
only with
true breeding plants. He also only measured absolute characteristics
such as
color, shape, and texture of the offspring. His data was expressed
numerically
and subjected to statistical analysis. This method of data reporting
and the
large sampling size he used gave credibility to his data. He also had
the
foresight to look through several successive generations of his pea
plants and
record their variations. Without his careful attention to procedure and
detail,
Mendel's work could not have had the same impact that is has made on
the world
of genetics.

Diagrams

In
cross-pollinating plants that either produce yellow or green peas
exclusively,
Mendel found that the first offspring generation (f1) always has yellow
peas.
However, the following generation (f2) consistently has a 3:1
ratio of
yellow to green.

This
3:1 ratio occurs in later generations as well. Mendel realized
that this
is the key to understanding the basic mechanisms of inheritance.

It
is important to realize that in this experiment, the parent plants were
homozygous
for pea color. That is to say, they each had two identical forms
(or alleles)
of the gene for this trait--2 yellows or 2 greens. The plants in
the f1
generation were all heterozygous. In other words, they
each had
inherited two different alleles--one from each parent plant. It
becomes
clearer when we look at the actual genetic makeup, or genotype,
of the
pea plants instead of only the phenotype, or observable
physical
characteristics.

Note
that each of the f1 generation plants (shown above) inherited a Y
allele from one parent and a G
allele from the
other. When the f1 plants breed, each has an equal chance of
passing on
either Y or G alleles to each
offspring.

With
all of the seven pea plant traits that Mendel examined, one form
appeared dominant
over the other. Which is to say, it masked the presence of the
other
allele. For example, when the genotype for pea color is YG
(heterozygous), the phenotype is yellow. However, the dominant
yellow
allele does not alter the recessive green one in any way.
Both
alleles can be passed on to the next generation unchanged.

Mendel's
observations from these experiments can be summarized in two principles:

The Principle of Segregation

The Principle of Independent
Assortment

Mendel
came to four important conclusions from these experimental results:

1.The
inheritance of each trait is determined by "unitsÂ" or "factors"Â (now
called genes) that are passed on to descendents unchanged.

2.An
individual inherits one such unit from each parent for each trait.

3.A
trait may not show up in an individual but can still be passed on to
the next
generation.

4.The
genes for each trait segregate themselves during gamete production.

Mendel
and Darwin

While Mendel knew of Darwin's
work (though
Darwin was evidently not aware of Mendel's work), Mendel's
ideas on heredity and evolution were fundamentally opposed, in certain
key ways,
to those of
Darwin. 2,5

"In a letter to William Bateson written in 1902
by Mendel's nephew, Ferdinand Schindler, stated, "He [Mendel] read with
great interest Darwin's work in German translation, and admired his
genius, though he did not agree with all of the principles of this
immortal natural philosopher" (Orel, 1996, p. 188). Bateson
(1913, p. 329) wrote, "With the views of Darwin which at that time were
coming into prominence Mendel did not find himself in full agreement."5

Now,
this isn't to say that there isn't a great deal of controversy in this
regard. Arguably most past and present authors and scientists
view or
viewed Mendel as a supporter of Darwinism. By
contrast,
Olby (1979, 1985) studied the historical context of evolutionary
thought during
Mendel's day and determined that Darwin's "'views on the role of
hybridization in evolution were very far removed from Mendel's'".5

"The extreme disagreement
among scholars about Mendel's view of Darwin's writings is probably
because Mendel wrote very little about Darwin, and thus most claims are
suppositions about what Mendel must have thought about Darwin. In his
surviving writings, Mendel's overtly referred to Darwin only four
times, all in 1870, four years after the publication of "Versuche" One
reference is in Mendel's (1870) Hieracium paper and three are
in his eighth and ninth letters to Nageli (Stern and Sherwood, 1966).
All four references are brief and reveal neither strong support of nor
opposition to Darwin's theories." 5

However,
in Mendel's copy of Origins, he did make occasional marks and
margin
notations. Mendel marked one passage where Darwin discusses the
uniformity
of hybrids in the F1 generation and the variability of their F2
offspring.
Darwin's explanation for this was that there was some alteration in the
reproductive system, some mutational effect. This explanation
differs
substantially from Mendel's explanation of independent assortment of
independent
traits or alleles. Also, Mendel directly contradicted Darwin's claim in
Origin
that changing conditions of life were the cause of variation in
domesticated
species. 5

In short, Darwin believed
in the inheritance of acquired characters.This led him to his famous theory of continuous evolution.Mendel, in contrast, rejected both the idea of inheritance of
acquired
characters (mutations) as well as the concept of continuous evolution.
The laws
discovered by him were understood to be the laws of constant elements
for a
great but finite variation, not only for cultured varieties but also
for species
in the wild.3In
his short treatise, Experiments in Plant Hybridization, Mendel
incessantly speaks of "constant characters", "constant
offspring", "constant combinations", "constant forms",
"constant law", "a constant species" etc. (in such
combinations the adjective "constant" occurs 67 times in his original
paper). He was convinced that the laws of heredity he had discovered
corroborated GÃƒÂ¤rtner's conclusion "that species are fixed with limits
beyond which they cannot change".And
as Dobzhansky aptly put it, "It is...not a paradox to say that if
someone
should succeed in inventing a universally applicable, static definition
of
species, he would cast serious doubts on the validity of the theory of
evolution."

As
the Darwinians won the battle for the minds in the 19th century, no
space was
left in the next decades for the acceptance of the true scientific laws
of
heredity discovered by Mendel.Further
work in genetics was continued mainly by Darwin's critics. In agreement
with de
Vries, Tschermak-Seysenegg, Johannsen, Nilsson, et al., Bateson stated:

"With
the triumph of the evolutionary idea, curiosity as to the significance
of
specific differences was satisfied. The Origin was published in
1859.
During the following decade, while the new views were on trial, the
experimental
breeders continued their work, but before 1870 the field was
practically
abandoned.In all that concerns the
species the next thirty years are marked by the apathy characteristic
of an age
of faith. Evolution became the exercising-ground of essayists. The
number indeed
of naturalists increased tenfold, but their activities were directed
elsewhere.
Darwin's achievement so far exceeded anything that was thought possible
before,
that what should have been hailed as a long-expected beginning was
taken for the
completed work. I well remember receiving from one of the most earnest
of my
seniors the friendly warning that it was waste of time to study
variation, for 'Darwin had swept the field.'" 4

The
general acceptance of Darwin's theory of evolution and his ideas
regarding
variation and the inheritance of acquired characters are, in fact, the
main
reasons for the neglect of Mendel's work, which (in clear opposition to
Darwin)
pointed to an entirely different understanding of the questions
involved.1